text:
What protocol should be followed after accidentally entering restricted airspace with a UAV?
This is related to my earlier question.
Delhi has a lot of areas with restricted airspace and no fly zones. Assuming we are able to get the necessary licenses and permissions to fly, what should we do if we accidentally enter no fly zones while in controlled flight?
The altitude will be less than 400 meters.
Once (and if) we are clear of the no fly zone, where should we report the incident?
This is a hypothetical question. I do not intend to break any laws knowingly.

Well this one I can answer even without being an expert on India's airspace regulations: Entering a "no-fly zone" or other restricted area with an aircraft - even an unmanned one - is going to be some kind of a violation.
You local authorities could tell you what kind, but it doesn't matter: it's a situation to be entirely avoided.
You should plan your test flights to occur far, far away from such restricted areas (as well as from airports, population centers, "open-air gatherings of people" (to use the FAA's parlance), or anything else that you might disturb, damage, destroy, or annoy during your testing.
This is almost certainly what the DGCA is going to tell you if you contact them and ask for advice on test parameters for an Unmanned Aerial Vehicle, but more importantly it's just plain good old fashioned Common Sense!
If you cannot exercise such common sense you should rethink & rework your plans until you can.
If after real, honest rethinking you're still in a situation where you feel you may inadvertently penetrate controlled or restricted airspace then coordinate with the agencies that handle that airspace PRIOR TO YOUR TESTING to make appropriate arrangements.

Now if you exercised all due common sense and still the absolute worst happens (Murphy's law rears its ugly head and LITERALLY everything that can go wrong does - even the engine cutoff and the emergency self-destruct didn't work!) you may still accidentally wander into airspace you shouldn't be in.
If that DOES happen your procedures should be effectively the same as they are in a manned aircraft anywhere in the world if you encounter the same situation: Communicate, Confess, and Comply.
Contact the controlling authority (e.g. tower, approach, the military division responsible, etc.), let them know what's happened, tell them where you are (in this case, where your drone is - they'll likely ask you to copy a number if they want to find out where "home base" is), and comply with any instructions they give you.
(This may mean you need a radio and a list of frequencies on the ground with you - picking up a sectional chart would be a good idea. You may also need a radio-relay in your UAV depending on what kind of range the vehicle has...)
Yes, the controller might get a little snippy with you (you are after all somewhere you're not supposed to be. Controllers tend to not like that!), and you might get in a little trouble for the violation (because again, you're somewhere you're not supposed to be), but that's all preferable to the potential alternative of smacking your UAV into a plane full of people at 300 feet on short final to Indira Gandhi International Airport.
As with all aviation operations, safety must be your overriding concern.

The restricted airspace (that I am talking about) in urban Delhi is primarily VIP residences such as the President's Estate, Prime Minister's Estate and Office, Minister's living accommodations, etc. There are lots of open areas and population density is probably the lowest in these areas. We understand safety. My main question was what should be done immediately when we realise we have entered such an area. Maintain straight course? Maintain altitude but change direction? Contacting any authority will take much longer than any of these immediate steps.

IMHO the answer isn't a what, its a don't -- any answer other than "don't" is inherently wrong, and any operation that puts you at risk of this sort of violation is probably a Bad Idea. If you want a "what" though, ask yourself: "What would the procedure be in a manned aircraft?" In my (US) training I was generally taught that if something like this happens I should "exit by the most expedient route" (which is usually a best-rate 180-degree turn), while attempting to implement "Communicate, Confess, and Comply".

Incidentally, if India has something equivalent to NASA's ASRS program reporting the incident to that program would be an important thing to do as well -- I didn't see anything equivalent on the DGCA website, but that's a pretty massive site :)

What would the procedure be in a manned aircraft? I wouldn't know specifics, (I am not a pilot) which was the whole reason behind the question :)

@AsheeshR The procedure any flight instructor would teach you is "don't" :-) But if you do, what I mentioned in my comment above is the advice I always got - get out by the fastest route, while trying to talk to someone to tell them what you did. (The assumption in manned aircraft being when you get someone on the radio and explain what happened they'll help you keep it from happening again until you get yourself sorted out.)


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Is a dancing crew dangerous?
Recently, the crew of an Indian airline performed a short choreographed dance sequence mid-flight on the occasion of Holi. This is, a not so rare practice amongst low-cost Indian carriers, who organize such dance sequences to celebrate special festivals (since festivals are a huge part of Indian culture, plus publicity for the airline afterwards).
Here is a YouTube video.

One of the pilots can be seen recording the dance on his camera.

SpiceJet specially planned this event, and had extra cabin crew on-board the flight as a precaution. Also, during the dance, one of the pilots was in the cockpit while the other one was outside, following standard regulations.
DGCA however, got wind of this, and turned out to be not in a very festive mood.

Mid-air Holi celebrations aboard eight flights have cost SpiceJet heavily, with the DGCA issuing show cause notice to the airline and suspending two of its pilots.

Here's another news link.
One of the arguments made by the DGCA was that this was dangerous to flight operations, and could have resulted in disaster had there been any emergency on-board during the event. Also, that this could result in an unruly cabin environment where in-flight safety of the crew and passengers would be affected.
Were the actions (suspension of pilots and showcause notice to the airline) justified? Is a dancing cabin crew dangerous to flight operations?

Hang on, was this in-flight or pre-takeoff?

Two years later... Holi is celebrated before takeoff ;-)

Well I suppose the answer depends on whether or not the flight crew are good dancers: If my uncoordinated self was trying to bust a move in an airline cabin I'd probably wind up kicking someone in the face, but these folks seem to be doing OK.
I can only assume dance lessons are part of SpiceJet's crew training program :-)

Alright, in all seriousness - there is increased risk any time every person onboard isn't securely strapped into their seats. There have been incidents (recent ones) where flight crew walking about the cabin have been injured due to unexpected turbulence, and those same unrestrained people can essentially become human projectiles to injure others onboard.
That said I personally don't think the cabin crew doing a mid-flight dance routine poses a significant safety risk.
The fact that one of the pilots was out of the cockpit recording it is slightly more questionable, and may be what has the DGCA upset in this situation. While there are procedures for having one pilot off the flight deck those provisions are generally for necessary things (like a trip to the bathroom, or investigating a systems fault). The intent is for the cockpit to be manned by two qualified pilots at all times.
If an emergency were to have happened during this dance routine the pilot we see in the video would have had to get back to the cockpit, strap in, and put on a headset (& possibly an oxygen mask) before being of much use.

Yeah I mean imagine if the co-pilot passed out. His mate is not there to deal with that, and for no "good" reason.

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Why doesn't the aviation industry use SI units?
This is a followup to What is the measurement system used in the aviation industry? and related to this question from History.
I can understand the arguments as to why adoption of SI units would not make sense for the general population, but aviation is a specialised business. All professionals are highly trained, and would (should) be well versed in both systems anyways, so the transition would be much simpler from the point of view of human factors. The technology would probably be much harder to shift but again, with more and more displays and documentation going digital in cockpits, this again seems to be a not so huge problem. Maintenance and manufacturing is again fairly specialised and restricted to a smaller number of companies as compared to the general case.
What are the historical factors that lead up to the adoption of imperial units in the industry? Why are they still being used when widely accepted scientific standards exist?

possible duplicate of Using feet vs meters for altitude?

@flyingfisch This is a more general question about the units system being used for altitude, fuel, length, ground speed, air speed, pressure, and also by on-ground maintenance in terms of tools and measurements.

ah ok, understood. ;-)

possible duplicate of What is the measurement system used in the aviation industry?

I've added answer to the linked question that attempts to answer this one as well. Hm, I'll probably copy it here.

Non-SI is only used for altitude, distance and speed except in US and some other American countries.

Altitude is in feet because 1000 ft happens to be reasonable vertical separation and 1000 ft is easier to calculate with than the corresponding SI figure 300 m. Also the procedures for instrument flying were first developed in the USA using feet.
Distance is in nautical miles because it is related to the unit used for measuring latitude and longitude. 1 nautical mile corresponds to 1 minute latitude (and longitude on the equator), which makes it easier to calculate distances from navigational maps using the grid lines as scale (large area navigation maps need to preserve angles, so they can't have constant scale). If angles were converted to decimal, 1 km would be 1/100 gradian. Alas, angles and time were never converted to decimal.
Speed obviously based on the distance unit in use.

Nevertheless if it was not for the prevalence of US-built planes after WWII and more advanced state of aviation in the USA at that time, we would probably be using metric in Europe too as continental planes before WWII usually had instruments in metric.

Well, approximately 1 gradian, at least. The reason that angles were never generally converted to gradians is probably because radians are much, much more useful for the vast majority of purposes.

@reirab: Radians are different thing. Still does not explain why gradians did not replace degrees.

Yeah, they are different for sure, but with engineering and science using radians for most purposes anyway, there probably still wasn't nearly as much incentive for them to push the switch than with most other units of measure.

One practical issue with radians is that they are irrational. A degree is approximately the daily change in angle to the sun, I would guess that it is deeply buried in our psyche :-).

@copper.hat: It's rather that in the area where it would help, navigation, backward compatibility was more important than warts of the system as it would mean redrawing all the navigational maps and that was a lot of work back then. Plus for all the warts of the sexagesimal system it has great advantage of having many integral factors.

With grads we would have to change to the one-in-sixty-four rule (which has a nice binary ring to it).

The nautical mile and derived units are the only unit system based on the earth being spherical. As such, they are more accurate both in units calculations and human factors. If you ever have to plot manually on a map with a divider, the ease of the nautical mile becomes readily apparent.

@TechZen: The fact Earth is spherical has absolutely nothing to do with this. The coordinate system used for specifying position does, as the text already explains. Both nautical miles and metres are derived from such systems (so nautical mile is not the only one), alas different ones and the one metres are derived from never caught up. As the answer already explains as well.

@JanHudec: Degrees > Gradians because of numerics. (Summary: 360 is "better" than 400 from a numerical standpoint, especially if you're going to break it up into parts -- which you're doing all the time with degs / grads.) Similar holds true for time -- nice "decimal" numbers might look better, but the current (Sumerian) 60 * 60 * 24 is better for subdivisioning.

@DevSolar: Yes, that is probably the main reason why it stuck for civil time (and physicists don't care, because they can simply use seconds and ignore the larger units). But for angles you don't really need division to equal parts much, so I suspect not wanting to redo maps is more likely reason there.

@DevSolar: Sometimes I am indeed wondering why we are not using base-12 instead of base-10. Because while using fingers for base-10 is more obvious than using them for base-12, they can be used for base-12 too (4 fingers times 3 parts, the other hand works as index) and the additional divisors are often useful.

@JanHudec: We don't usually need the subdivision-features that base-12 would make (marginally) easier, so the benefit of switching to base-12 (compared to the royal PITA it would be in everything from language to engineering to computing) is negligible. We already had to cope with the effects of base-2, and the not-quite-SI Ki, Mi and Gi bastard prefixes. ;-)

@JanHudec - I disagree, The fact that the earth was sphere did play a role that is why it is one nautical mile was defined as one arc minute along any meridian, a measure meaningless on any surface other than a sphere. Your definition is later, hacked on version. The nautical mile, like many pre-metric system, evolved out of the nature and scale of the phenomena observed as well the angle being the only form of accurate repeatable measurement. It worked fantastically back when most calculation where done by various divisions and ratios instead of numerically.

@TechZen: The fact that the Earth was a sphere did play exactly the same role in why meter was defined the way it was, as a 1/100th of a gradian along any meridian. The units are both geocentric. The distinguishing feature is which matches the coordinate system of maps we use. And the only reason we use that particular coordinate system is historical; any coordinate system would make just as much sense, but long ago we arbitrarily chose one and it stuck.

You state that "the transition should be much simpler in terms of human factors."
Ideally, yes.  In practice, no.  When mistakes happen, they may well be lethal.  The  Gimli Glider incident was partly caused by a confusion between which system was used to measure fuel quantity.  From Wikipedia:

The subsequent investigation revealed a combination of company failures and a chain of human errors that defeated built-in safeguards. Fuel loading was miscalculated due to a misunderstanding of the recently adopted metric system which replaced the imperial system.

The fact that the Gimli Glider incident ended with no loss of life doesn't negate my point: Human factors such as fatigue will find a way to defeat the best plans of the Safety Engineers.  One of the human factors of which you speak is "what you learned first, you learned best" — and this learning is likely to predominate in an emergency situation.

This is a historical development that dates back to that much of the early aviation equipment was sourced from the United States, and was consequently in imperial units. This in particular occurred after World War Two, and hence mixing them was a bad idea, and the imperial system stuck. Interestingly, the places where the US did not have a lot of influence- the former USSR and China for example- use metric.
As for airspeed in Knots and distance in Nautical Miles, this comes from aviation's nautical heritage.
All pilots can do the conversion, but it's rather that changing all the instruments from the imperial system to metric system that would cause a huge headache, among other things, and the cost would outweigh the benefits.

Instruments would have to be changed.
All reference material would have to be converted.
All maps/charts/etc. would have to be converted.
Pilots would have to become accustomed to the new units and to learn the changed values which they will probably have memorised, such as important airspeeds.

As it currently stands, pilots have to regularly switch the units during flight. Russia, CIS and China are not exactly negligible part of the world.

Note that the US does not use imperial units (or, as Americans often call them, English units): it uses US customary units. The US fluid ounce is a few percent different from the imperial fluid ounce but a US pint is only 16 ounces compared to 20 in imperial; similarly for multiples such as quarts and gallons. Also, a US hundredweight is 100 pounds, whereas an imperial hundredweight is 112, with a concommitant difference in the definition of a ton.

And air traffic controllers would have to become accustomed as well. The transition would surely result in more than a few improper headings.

I would say that the conversion issue isn't the main issue at all. It's a big issue, but I think the larger issue is that feet, nautical miles, and, consequently, knots are objectively more useful/convenient units for aviation, for the reasons Jan Hudec listed.

The first part of the answer is that it is too difficult to change. It would involve lots of rewriting rules and manuals and retraining.
The second part is that it does not really matter. As a pilot you (or I) do not really care how height or speed or whatever is measured. In the procedures we follow it says height 2000 and speed 200 (or whatever). And the units are the same as the instruments are calibrated in. If you the units where different, the instruments and the procedures would show different values. As long as they are in the same units it would not matter.

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What is turbulence, and how does it happen?
I'm not a pilot, so I always wonder when I'm flying why does it happen... Is it because of bad weather, or something else?

What is turbulence?
Turbulence is

random and stochastic
Each realization of a turbulent flow is unique and fluctuations in velocity are very irregular in space and time.
dissipative
Turbulence cannot maintain itself and will decay into laminar flow without energy input from the environment (e.g. mean shear in the velocity field or buoyancy). Reducing viscosity in a flow does not remove dissipation, it just moves dissipation to ever decreasing scales.
diffusive
A time-averaged scalar transported in a turbulent flow will have larger plume than that of a laminar flow.  For example, if you injected a dye into a smooth flowing river and one into a turbulent flowing river, the dye plume would cover a larger area in the turbulent flow.
three dimensional
3D vortex dynamics play a very important role in turbulence and for most cases you cannot represent a turbulent flow as a 2D problem.
occuring on a wide range of scales
This is perhaps the most important distinction.  When I say scale, I am referring to a characteristic length and velocity of an eddy, or swirl.  Turbulence is produced at larger scales and the transfers its energy to smaller and smaller scales.  At some (very small) scale, dissipation starts happening and the turbulence decays.  A feature of a turbulent flow is energy at all scales, which basically means there are whirls of all sizes and it is the interaction between all of these swirls that gives turbulence.

How does it happen?
The main production of turbulence is through mechanical production and buoyant production.  Turbulence happens because energy is being put into it via these production methods.  Turbulence will decay on its own but will not stop until production ceases.

Mechanical production can be envisioned as a shear flow, or air of two different speeds interfacing.  This can also be wind against a wall, or around some object.  The velocity gradient at the periphery of the jetstream is a shear flow, and the boundary layer that we live in is a sheared environment on most days.
Buoyant production is through buoyancy.  Relatively warm parcels rise and this is balanced by subsidence and if you follow these thermals and the descent between them and squint hard enough, you can envision eddies on the largest scales.

Both of these methods put energy into turbulence at large scales.  For boundary layer flows, this can be on the order of a kilometer or more, typically the height of the boundary layer.  These big eddies will have perturbations of their own and these smaller perturbations are the next smaller scale.  Energy is cascaded downward until the sizes of the eddies are small enough that viscosity dominates over inertial forces and the energy is then lost from the turbulence as heating.  The superpositioning of these eddies at all scales resembles a jumbled bumpy mess, and this is precisely how we experience turbulence.

Splendid answer!

And how common are these phenomena at cruising height? In India, I notice it only "below the clouds". I have flown mostly to plains/coastal cities.

@aitchnyu While turbulence is fairly ubiquitous in the boundary layer (generally "below the clouds") you'll find it all over the place.  Any cumuliform cloud is going to contain turbulence (buoyant and mechanical production) and you'll find turbulence anywhere large gradients in wind are present (e.g. frontal zones, the periphery of the jetstream particularly if a strong jetstreak is present) through mechanical production of turbulence.  Orographic forcing will also produce some impressive turbulence in the lee of the mountain and downstream aloft.

Air turbulence is very much the same as the currents of a river.
When a parcel or stream of air moves differently than the area around it, you get turbulence as you transition between them.
An example would be if you are outside on a windy day but stand behind a tree to "get out of the wind".  If you step out from behind the tree, you will feel a sudden "jolt" as the wind hits you.  If it's a particularly strong wind, it could even cause you to lose your balance for a second until you compensate for it.  Well, these invisible air currents affect an aircraft in much the same way.
It isn't normally dangerous, and I think of it like driving down a bumpy road.  Of course, it's usually even safer in an airplane because even if you hit a big "pothole" you don't have to worry about it throwing you off the road into a tree.
The actual movement of the air can be caused by several things:

Wind hitting something (like a building or a mountain) and displacing it (just like rocks in a river).
Air being heated by the ground which then causes it to rise.
Weather activity like fronts, thunderstorms, or jetstreams.
An aircraft moving through the air will cause turbulence of its own (known as wake turbulence).


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Emergency landing at Air Force airfield
What would happen if I perform an emergency landing at an Air Force airfield?
Would they send fighters planes to intercept me?
Would they arrest me?
Would they detain my aircraft?

Nope... I want to know what would happen if i land there without permission. In that question he's asking what he should do to get that permission.

not a direct dupe but the accepted answer states that emergency situations will allow you to land.

In short: In emergency situations you are permitted to do almost everything, including landing at any available airfield. In most cases you have either: the TWR frequency set, the APP frequency set, a FIS frequency set or the international distress frequency (GUARD) set and announced your intentions.

There are some bases that are joint-use anyway, and in that case, it would be almost exactly like an emergency landing at a civilian field. Then most bases will do little if you are truly an emergency. The times this would come into play are sensitive bases such as Nellis AFB or Groom Lake, or bases that have significant national security interests, such as Andrews AFB, or potentially a base with nuclear bombers.
In these cases, you would probably get a fighter escort if you reported intentions soon enough or entered restricted airspace. Most bases will not provide a fighter escort, as there is little they can do to help, and you should not be a threat to anything from the air.
If you declared an emergency and nothing was actually wrong, you would almost certainly be arrested. If there was an actual emergency, you would probably be detained at a sensitive base, but at most bases they would just keep you confined to unsensitive visitor's areas. Here is an incident where the pilot was forced to land at Travis AFB (which only has C-5, C-17, and KC-10 aircraft). The pilot followed emergency procedures and everything turned out well. But the report notes:

"Had he not made contact with RAPCON, security forces would have to respond as if the aircraft was hostile," Wilson said. "His proper communication allowed us to clear the area for him and notify the appropriate agencies."

As your aircraft is not flyable at the moment, it would have to stay there until it was repaired, but in most cases you should be able to fly it out after it was fixed. In a sensitive area, I have heard reports of airplanes being disassembled and shipped off the base, but nothing concrete about it.
However, at most of the most prohibited areas the prohibited airfield is rarely going to be you best option (e.g. it would be unlikely for Groom Lake to be your only option, unless you were snooping around the edges, which they would not take kindly to). But just remember - in an emergency, you are in control until your plane is on the ground. So make
the conservative decision and land safely. If it is an emergency, and you weren't pushing limits before the emergency, the most you should get is a stern talking-to and a short detainment.


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Could you land a large airplane on short circular runways?
Considering that building runways is expensive and that they take up a lot of space, could you land a large commercial airliner on a circular runway, braking as you go around the curves?

"Breaking" is the most likely result, yes. :)

I was thinking of airports like Berlin-Tempelhof, which is in the  middle of the city and therefore small, but which has circular taxi-ways on the perimeter around the runways. One reason this airport was closed was that the runways didn't support large aircraft.

Circular runways are called helipads but they work better for helicopters.

Circular Runways: Because if there is one thing I want during my landing roll is to turn away from the headwind

The short runway problem has already been solved: http://en.wikipedia.org/wiki/Aircraft_arresting_system. Brace!

So that would be up/cross/down/cross/up-wind take-offs and landings all the time...
@SebastianHenckel: Only if the circular runway would be all around Berlin, outside ring-bahn circle...

http://www.endlessrunway-project.eu/ you aren't the first to think of this

Reminds me of the old joke (from the early Fifties, and inspired by underpowered aircraft like the F-84): If someone builds a runway all the way around the world, Republic will design an aircraft that needs every inch of it.

To my surprise, no on has mentioned the fact that planes have different turning radii that would change based on each aircraft's airspeed and the current windspeed.  Pretending that there is a "one size fits all" radius is a huge fallacy in this idea when they talk about "banking the runway" an therefore, also the aircraft.

@IgbyLargeman: Not for commercial airliners.

Since there are actual projects for things like this I guess it is possible at least in principle:

https://www.endlessrunway-project.eu/
From the site:

The main feature of the circular runway is that it will become
  possible to let an aircraft operate always at landing and take-off
  with headwind. Whatever its strength and direction, the Endless Runway
  becomes independent of the wind. When allowing  limited crosswind,
  airspace users can shorten the global trajectory of the flights
  through optimized departure and arrival routes.

Apart from that they seem to hope that they can start/land more airplanes per time unit by shifting the start/landing point thus avoiding the problem that airplanes must not follow other airplanes to close along the same trajectory.

yes, this thing would have a runway be a very large circle, with a radius large enough that you can always run along it (nearly) straight and thus always have optimal wind. In reality of course that'd be so large as to be impractical. But it's essentially possible using a circular platform that's 10x10km in size, not dissimilar to the meadows used for WW1 and early WW2 airfields where aircraft could take off and land on a roughly circular patch of grass around the periphery of which was constructed the airfield.

They envisage a circular runway 3000 m in diameter. By my back-of-the-envelope calculations, the lateral acceleration necessary for following that circle at a takeoff speed of 140 knots would correspond to a bank angle of about 20 degrees. Getting airborne will be exciting ...

How does one calculate/estimate the bank angle?

@Jens: It's arctan(speed²/radius/g)

Not to mention that, at a 3,000 m diameter and 140 kt takeoff speed, the wind would be changing direction relative to your aircraft very quickly. Where there really becomes a problem is as you're turning through the area of the runway where you have a headwind. As you turn into that area, your airspeed increases, potentially to the point that you leave the ground. As you turn out of that area, you're airspeed drops again, potentially stalling the wings. Bad news.

@reirab why would you keep turning after lift of?

@JensSchauder, 'Lift off' is not something that happens instantly. When you're taking off, you start pulling back at the rotation airspeed and the nose starts to rise into the air then. However, the main gear doesn't rise off the ground for a few seconds after that. On a circular runway with a 1,500 m diameter near V2 of an airliner, you would do some significant turning during that time. Also, even once you're fully off the ground, it takes a finite amount of time to roll out of the turn. You can't stop turning immediately.

I downvoted this because compared to other answers, this answer fails to address in detail the ins and outs of the proposed runway.  Crazy engineering designs are proposed every year, but that doesn't mean they're practical.  I'd like to see more discussion of advantages, whether those advantages could actually be realized in practice, and disadvantages of the proposed design.

The last time the endless runway site was updated was March of 2015, so it looks pretty dead as a project.

@zeta-band every few month some news flunky looking for something "sciency" for the evening news comes across the site and interviews the people involved about their "revolutionary idea" that they're going to implement "real soon now" (never mind that it was tried and rejected over 50 years ago).

As a matter of physics, there's a certain amount of braking and steering force available to the aircraft after it lands.
Suppose the aircraft touches down moving due north, and continues clockwise around your circular runway. After a quarter-circle it travels due east -- but that means that enough braking force has been applied to reduce the northward component of its velocity to zero, while also giving it some eastward velocity.
If only that eastward force hadn't been applied, the aircraft would be at a halt now -- using less force than what's necessary for it to follow the circle.
So if the plane can keep itself on a circular runway, it will be even easier for it to come to a halt on a straight runway whose length is the radius of the circle.

(Also, the plan ignores that the critical factor in the length of runways is not landing but taking off -- there must be enough runway to brake down safely if the takeoff is aborted just before rotation. And taking off from a curved runway sounds even crazier than landing. If the runway is horizontal, the plane would become airborne in the middle of an uncoordinated turn...)

What if the runway was slanted? :P

The logic seems reasonable, but is it actually true? That is, if a vehicle in general can make an unbanked turn with radius r, then it can necessarily come to a halt in distance r?

@SteveJessop: There are some differences being swept under the carpet here because a turning force does no work whereas a braking force has to dissipate some waste heat. But it looks like the limiting factor in actual fact is not energy dissipation but the available friction between the landing gear and the runway anyway, so yes, it ought to be true.

@anarcat: What if a straight runway went uphill? Oh, you mean for takeoff? A banked curved runway would be right for only one specific Vr, which I imagine would be operationally cumbersome. :P

@SteveJessop that might actually be a very interesting question to pose over on [physics.SE].

@HenningMakholm Well, you could have a runway with a U-shaped profile, so that every Vr can choose a slant angle right for them. Although I imagine the pilots will complain even more about this.

Also, the question assumes that braking and turning is done by the same force, but isn't an airplane better at turning than stopping because it can use the tail rudder which it wouldn't be able to use for braking?

@anarcat many runways are slanted, slightly and sometimes not so slightly. Makes takeoff or landing slightly shorter or longer depending on the angles involved, but rather hard to reverse if the wind shifts :)

@Superbest at the low speeds involved, the rudder is not very effective at turning, so you'd have to rely on differential braking and thrust instead (talking airliners here, not your C172).

@jwenting: Differential braking/trust produces no sideways force -- it merely yaws the aircraft, and actually forcing it to move away from a straight line right depends on wheel/runway friction after the fact. And it's even worse for the rudder -- since the rudder is aft of the CoG, yawing the aircraft to the right depends on producing a net left force on the tail and vice versa. Again the actual centripetal force must be supplied by the wheel/runway interface.

@HenningMakholm: Um, no, yawing the aircraft to the right does in fact help to turn it, by putting the aircraft into a sideslip and allowing its weathervane stability to shift its direction of flight.  Otherwise, it would be impossible to make an unbanked turn in the air, when this is actually easily done (although somewhat dangerous due to the risk of causing a spin).

One major complication would be the shift in relative wind as you work your way around this curved runway.
On approach, the wind would be coming from one direction (ideally straight head), then as you touch down and begin your curve while slowing down, the wind shifts direction to come more from one side.  So now you're trying to stay on a centerline that is curving, at somewhat high speed, while the wind moves to one side of your plane.
That is just insane.

Another major issue would be the amount of "float".   Although airplanes target touching down right at the end of the runway, that doesn't always happen.  Sometimes due to excess speed, gusty winds, or a misjudged flare, the plane will float a distance down the runway before actually touching down.   During this time, the plane's nose may be high, and visibility of the runway is limited.  Because the runway is straight, it's no problem.
But suggesting that a pilot navigate a curved runway while floating further than anticipated is just nuts.  If they don't turn enough, they end up outside the curve of the runway.  If they turn too much, they end up inside the curve.  Either way, if they overcorrect at low altitude and airspeed, the chance of a catastrophic crash is unacceptably high.

A circular runway maybe works but this circle can't be small as the turn must be really smooth, talking about just a few degrees per 1000 feet. So this runway would actually be very "long" and need a lot of space.
Let's stop thinking about that! You would probably not be able to maintain 'directional' control if the outboard engine fails (e.g. left turning runway, right engine fails). How would you set up an ILS approach? Aircraft aren't build for this. It would be dangerous or even impossible and would never be approved.

This would also be disastrous with anything less than great braking action.  It would also destroy our ability to do reduced vis (300 or 600 RVR) takeoffs.  Where would the approach lights go?  Would there be an extension like a running track?

Adapting ILS would certainly be a challenge, but even now certain airports have complex approaches. Curved path approaches have been demonstrated before, and with GPS there are now even more possibilities.

The problem with some of these proposed ideas (like a treadmill) is that they don't account for the inertia of the plane. You can't take a 500,000 lb 747 travelling at 160Mph and simply place it on a treadmill or spinning runway. There is still forward velocity to account for. Even if you could find a way to make the aircraft stop "very fast" without ripping the landing gears off (ie. the arresting gear suggested by Dan), think of the effect it would have on passengers. Consider going from 160Mph to 15Mph in only a few seconds. It's not comfortable, and for many people (think infants and elderly), it's not even safe.
In regards to the aircraft turning, commercial airplanes are quite top-heavy, at least when compared to cars. Cars can turn very fast, even at high speeds (think NASCAR), because they are low to the ground. They have huge surface area on their tires in relation to the size and weight of the vehicle. Airplanes are the opposite. Compared to their size and weight, landing gears are very small, and not very useful for much else than rolling in a straight line.

As a pilot, it sounds terrible.  Even landing or taking off on a strip with a hump in the middle, so you cannot see the other end, dramatically increases the workload, and the degree of risk.  Not to say that it cant be done, when everything goes right, but if anything fails, then you raise the risk of a crash enormously.
And at the moment, the bottleneck is not getting to the airport, its waiting around to go through all the procedures when there that takes most of the time.  So its not much advantage to put an airport nearer the city anyway.  A better road, catering for airport traffic only, to further away, may well cut the transit time.
Personally I would rather land an aircraft somewhere where there are as few obstacles as possible.  Gives a much better chance of success when things go wrong.

Yeah, most larger cities have tall enough buildings that you don't want the airport too close to the city center, anyway.

The cost of building a "circular runway" (probably with banked sides like a Velodrome)  would probably be more difficult and expensive to build than a big long flat bit of concrete.
Another way to solve the same problem would be to use arresting gear and catapults like they do on aircraft carriers (which have the same problem - too expensive to build a big enough runway).
I'd pay extra to land and take off on such a runway, but I may be in the minority :-)

A banked runway sounds like a recipe for grounded wingtips...

@DavidRicherby - not if you're very very careful :-)

As many of these other answers speak to as well, the rate of turn for aircraft on the ground is very low, so the circle would have to be far larger than practical.
However, on a very related note, early airports all had circle landing areas. From wikipedia:

The earliest aircraft takeoff and landing sites were grassy fields. The plane could approach at any angle that provided a favorable wind direction. A slight improvement was the dirt-only field, which eliminated the drag from grass. However, these only functioned well in dry conditions. Later, concrete surfaces would allow landings, rain or shine, day or night.
  http://en.wikipedia.org/wiki/Airport

The landing area might have been circular or oval shaped, but the actual landings and takeoffs were straight.

... yes, I am aware. Thats the rate of turn restriction I mention. As explained in the quote, the advantage of the circle is not to shorten the field but to give the pilot the ability to always land and takeoff into the wind.

Yes.  The "long line loiter" method (see https://en.wikipedia.org/wiki/Pylon_turn ) could be used to lower a weighted line from the airliner to the ground.  The line would be retrieved by ground crew and attached to a winch mounted on a turntable on top of a tower in the middle of the runway circle, tall enough for the line to clear all obstacles.  The winch would be equipped with a system allowing it to apply constant tension to the line, which could be varied to equal any desired tension.  As the airliner descends, the winch would start applying more and more tension to the line, so that the line would provide some of the centripetal turning force, allowing both the bank angle and the turn radius to be decreased, after the manner of a control-line model airplane.  By the time the airliner reached the surface, the line would be providing all the centripetal force required for the airliner to stay over the circular track with zero bank angle.  As the aircraft approaches the surface, a computerized control system linked to a GPS receiver onboard the aircraft could be used to ensure that the line tension was modulated in such a way as to keep the aircraft nearly over the centerline of the circular track, despite variations in wind speed and direction, and then at that point the winch spool could be locked to keep the line length fixed until touchdown.   A similar method, but in reverse, could be used for takeoff, with the line being jettisoned (or simply released from the ground winch and reeled back up to the aircraft) after the aircraft is established in a "long line loiter" turn at the appropriate altitude and bank angle.

Different point of views can be derived of such a suggestion:

Huge turntable-like runways which turn while the landing aircraft rolls out. Turning at the same speed as the aircraft decelerates, result in the aircraft always be facing the same wind conditions during stopping process.
Development of the first item: a treadmill-like landing runway. At the time of touchdown the aircraft does not require any airstream along its wings. The following deceleration could be performed on a treadmill or assembly line.

But honestly, none of them would financially or even technically pay off the effort of just expanding existing runways in length.

A treadmill wouldn't help. If you landed without using wheel brakes, it would obviously take the same distance (relative to a stationary point on the ground) to stop, whether the runway was moving or not. So the only way you could stop in a shorter distance would be if you could dissipate more kinetic energy per second in the wheel brakes, i.e., by having more powerful brakes and grippier tyres. But then you could use those brakes and tyres just as well to get a shorter stopping distance on a static runway!

Sure, without wheel brakes it takes all the same distance. But was thinking about using wheel braking + treadmill for a decreased need for landing space on an airport.

The version without wheel braking was a thought experiment to set things up. With wheel braking, a treadmill wouldn't help because the plane would need more powerful brakes and stronger, grippier tyres or it would just skid against the moving pavement. To avoid that, you would need better brakes and tyres than existing planes have but, if you had that, you could stop shorter on existing runways, without needing the expense of the treadmill.

This will work if the circular runway is rotating.
The runway should be on a pedestal, and spin up to an to an angular velocity such that the tangent matches the aircrafts speed.  then rise up and catch the aircraft, slowly reducing angular velocity and lowering to the tarmac.
Take off would proceed in the reverse manner.  with the aircraft speeding up trying to stay on the rotating runway as it rises and increases angular velocity.
In the event of alien invasion we can use the rotating run ways to launch projectiles such as the airport busses.

I can't tell if this is a serious answer or not...technically it might work, but it's just so ridiculous it sounds sarcastic

Aircraft fly due to lift created from air moving over the wings, not because the ground moves quickly under the landing gear. Even if there was a rotating disc under the air craft, there'd be no lift created.

@CJBS, You're correct, of course, however, I assumed he meant that the aircraft would be stationary on the runway and let the runway accelerate it up to takeoff speed. Technically, that could work, but it would require far, far more energy input than just letting the aircraft take off under its own power (not to mention the obvious problems with trying not to get thrown off the side of the runway, changing wind direction, etc.)

@reirab - having a set of stationary fans blowing huge amounts of air at the craft is potentially more practical than having a spinning disc...

@CJBS, Well, that wouldn't really help anything, as it wouldn't actually accelerate the aircraft forwards. You could get some air flowing over the wings while it's behind the fans, but that's not going to help you once the aircraft rises out of the stream of air being blown at it by the fans. It would then immediately stall and plummet back down into the ground. Of course, the important thing to note here is that neither solution is practical. - lol

Great. We could eliminate the aircraft altogether and simply fling the airport buses, complete with passengers, luggage and chickens, in the direction of the destination.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Are commercial aircraft required to have the model number painted on them?
On a lot of aircraft, the model number / aircraft name is clearly marked somewhere on the fuselage, e.g. "Airbus A320". Is there any rule or mandate, that all aircraft must bear the model number at some designated place on its body, to help others identify them?

Can you provide an example?

Example: A-320. Though I know the aircraft type, on many airlines I see the name "Airbus A-320" displayed towards the rear of the aircraft.

As far as I know, that's more for advertising. The nosewheel doors often have the model for ground handler reference. Helps the marshaller be certain of where to stop it, the baggage guys what equipment to bring, I dunno? Stuff useful to rampers.

Every aircraft I have flown had a data plate mounted somewhere in the outside, showing the make, model, serial number, and the year of manufacture. You probably can't see it unless you're 3 feet away, but it's there.
The relevant regulation in the US is:

§ 45.11 Marking of products.
(a) Aircraft. A manufacturer of aircraft
covered under § 21.182 of this chapter must mark each aircraft by
attaching a fireproof identification plate that—
(1) Includes the
information specified in § 45.13 using an approved method of fireproof
marking;
(2) Must be secured in such a manner that it will not likely
be defaced or removed during normal service, or lost or destroyed in
an accident; and
(3) Except as provided in paragraphs (d) through (h)
of this section, must be secured to the aircraft fuselage exterior so
that it is legible to a person on the ground, and must be either
adjacent to and aft of the rear-most entrance door or on the fuselage
surface near the tail surfaces.

and

§ 45.13 Identification data.
(a) The identification required by § 45.11 (a) through (c) must
include the following information:
(1) Builder's name.
(2) Model designation.
(3) Builder's serial number.
(4) Type certificate number, if any.
(5) Production certificate number, if any.

Here's an example from a Stinson 108:

On airliners the data plate is often located inside near the entrance door. On most light GA aircraft it's affixed near the tail (the regs say "either adjacent to and aft of the rear-most entrance door or on the fuselage surface near the tail surfaces")

Could you provide a picture and a link to the relevant regulation (if any)

No. It's easy enough to find photographs online of aircraft that don't have the model number written on them in an obvious way. If there were any regulatory requirement to have the model number painted on the plane for recognition purposes, it would have to be obviously visible or there'd be no point.

ok, so there is no international regulation/convention to paint the model number on the aircraft

FAA AC-45-4 act is regulating that. Each airplane made after that year must have nameplate attach to the fuselage.

Welcome to Av.SE.  It would be helpful if you could include a link or other reference to this information; as the answer stands now, it amounts to "somebody on the internet claims..." which isn't a particularly useful answer.

@RalphJ Huh? It's right there: Federal Aviation Administration Air Circular 45-4.

I'd expect an ANSWER to actually link to the relevant section of the publication.

@RalphJ OK, that's a fair point. But I doubt a new user would pick that up from what you originally wrote -- I certainly didn't!


text:
Open source drone technologies
What is the best place to understand the details of drones?  Preferably something open source.  Books would also be helpful.

What aspect of drones are you trying to understand? At the moment, your question is essentially, "Drones: tell me all about them!", which is far too broad.

OP: Asking what the best of anything is pretty much ensures your question will be put on hold.

I can think of a couple of open source UAV projects:

Paparazzi is a complete system of open source hardware and software for Unmanned Aircraft Systems, including autopilots and a ground control and planning station. All of the hardware plans and software source code you need to make a drone from scratch are available on github. But not everyone has the equipment to make a printed circuit board at home.
Open pilot is an autopilot for UAVs. They also sell hardware if I remember properly, which might make it a bit easier if you're just starting.

If you just want a simple introduction to flying drones, DIY drones has a quick guide to the basics.


text:
What is the best way to reapply for a denied Class 3 medical certificate?
To Become a helicopter pilot is a major life goal of mine. My aunt was a naval pilot, when she took me up in a helicopter it was one of the most memorable experiences of my life.
Background: Today I have a job as a software developer at a successful company, I have graduated college in engineering. I work out regularly and eat healthy, I am able to complete 15 reps of pullups. I have excellent vision. My temperament is mild. I am reliable and prompt at work and I treat others as I would like to be treated.
Last year I was denied my class 3 medical license primarily due to being prescribed an antidepressant, which I was prescribed in February of 2015 as an off label treatment for ADD, I never took a single pill as the side effects scared me, also I am not depressed so it was a little off putting. There were complications due to my having received some counseling for alcoholism in college. In reality I made some mistakes in college and did not drink responsibly on a few different occasions, I have never been a regular drinker. It was possibly the new environment and the fact that I had no experience drinking. I have never been arrested, I have no DUIs or any of that nonsense. Receiving this news was very difficult considering the cost, time, effort and the fact that I did not even take any of the medication.
Currently I am not taking any medications or being treated for any issues. Job is going well, I experience a reasonable level of fulfillment and even volunteer on occasion. Getting my pilots license is a major goal of mine. Recently I decided to cut down on drinking, I now almost never drink unless it's with family or old friends. Realistically that ends up being one or two drinks less than once a week, I would not mind completely abstaining.
My primary question is: would it be worth it to again pursue the class 3 medical? Or will I be denied once more at this point? If this is the case, are there additional measures I can take to ensure that I get this license?

You really ought to seek advice from a certified aero-medical. Random people on the internet are not able to accurately estimate the outcome of your specific case. Good luck!

Join AOPA and ask their medical team and in their medical forum, and do not apply for a medical again without extensive consultation with a senior AME first. Things are complicated right now by the third-class medical reform going on and your history is problematic (I'm not judging, it's just a fact that alcohol, counselling and ADD are all major red flags at the FAA); once you have a rejection it's extremely difficult to get past that. It also closes off the option of flying as a sport pilot with a driver's license instead of a medical.

Take steps and do your homework BEFORE applying for your medical through FAA MedXPress!
I suggest discussing your concerns with a qualified Aviation Medical Examiner or beforehand, assuming you are a member, with AOPA Legal and Medical Services. AOPA has an excellent list of resources on health conditions that may affect certification.
Because a possible history of counseling may qualify as treatment, consider the alcohol and drug abuse guidelines. Note the phrase complete abstinence.

1. To be considered for medical certification, you should write to the FAA, stating that you have completed at least one full year of recovery with evidence of complete abstinence after being treated for drug or alcohol abuse. Although the Part 67 regulations require that there be a sustained period of abstinence of no less than the preceding two years, pilots may be considered for recertification as early as one year after successful treatment with appropriate documentation.
2. The FAA will require the following:
a. Your complete treatment records and discharge summaries.
b. An evaluation conducted by a professional who has had training in diagnosis and/or treatment of addictions. These professionals include substance abuse counselors, psychologists or psychiatrists, other physicians with training in addictive disorders, or members of the American Society of Addiction Medicine (ASAM). (A psychiatric and psychological evaluation may be required if the initial evaluation is inconclusive).
c. Evidence of successful completion of an inpatient or intensive outpatient program with a documented commitment to abstinence.
d. Participation in an acceptable aftercare program consisting of individual and group counseling sessions for at least 12 months.
e. Establishment of a monitoring system that includes a physician with expertise in substance abuse disorders, and
f. Additional monitoring reports from employers, family physicians, or others, as well as alcohol testing when indicated.

The mental health section has a page on ADD/ADHD. Summary excerpt is below.

Because of the potential for significant underlying psychiatric or psychological problems and the side effects of medications, the FAA does not currently allow medical certification for a diagnosis of ADD/ADHD requiring the use of medication.
For medical certification consideration, you will need to have
discontinued all psychoactive medications for at least the preceding
90 days.  After being off the medications for 90 days, a
neuropsychological evaluation should be conducted.  The
neuropsychological evaluation must also include the following three
tests:

Trail Making Test
Wisconsin Card Sorting Test
Paced Auditory Serial Addition Test (PASAT)

If the airman is an adult who has been prescribed one of the
stimulant-type drugs for ADD, but has not been diagnosed with the
disorder, the medications will have to be discontinued for three days
before undergoing the neuropsychological evaluation noted above.

Because you may have a history of treatment with antidepressants due to the past prescription, the mental health section has a page on special issuance for depression with antidepressant usage. Summary excerpt is below with emphasis in the original.

Effective April 5, 2010, the FAA began considering individuals for
special issuance medical certification who are being treated for
depression with one of four specific antidepressant medications.  All
classes of medical certification will be considered, but individuals
who are granted a special-issuance medical certificate under this
policy may take only one type of antidepressant medication limited to
the following four medications: Fluoxetine (Prozac), Sertraline
(Zoloft), Citalopram (Celexa), or Escitalopram (Lexapro). All these
medications are SSRIs, antidepressants that help restore the balance
of serotonin, a naturally occurring chemical substance found in the
brain.
Increasingly accepted and prevalently used, these four antidepressants
may be used safely in appropriate cases with proper oversight and have
fewer side effects than previous generations of antidepressants.
While the focus of this policy statement is on individuals being
treated for depression, the FAA realizes that these four medications
may be used to treat conditions other than depression.  It should be
noted, therefore, that, in all instances, the FAA will continue to
consider applicants and make determinations on a case-by-case basis
under the special-issuance process just as it always has.
…
No regulatory changes are being made under this policy.  Further, the
FAA continues to believe that applicants requiring use of multiple
antidepressant medications or use of any other psychotropic medication
in conjunction with any one of the four specified in this policy will
not meet the criteria set forth under this policy.  The use of
psychotropic medication continues to be disqualifying under the
medical standards and special-issuance certification will be granted
only after thorough analysis of each individual case presented and
only when appropriate conditions and limitations are in place so that
the applicant may safely be permitted to operate an aircraft.  It
should be noted that as new information becomes available and
recommendations from the medical community change it may be necessary
for the FAA to again revise its policy.


text:
What are the advantages and disadvantages of autogyro aircraft?
What are the advantages and disadvantages of autogyro category of aircraft?  Is there a maximum speed an autogyro can attain?

They do not tolerate lazy pilots. I know a few AG pilots, one is an accomplished instructor and one is a DE. There is still some disagreement about what is published in the FAA's "flying handbook." There are also some common AGs that my contacts simply refuse to fly for safety reasons.  Apparently there are very few truly competent AG instructors out there.

Autogyros survive just as fun aircraft, capable of very slow but safe flight and very suitable for recreational flight. Bensen-type gyros, with fixed-pitch, seesaw rotors, are –with very few exceptions– the only extant type. Concerning speed, the autogyro is limited (as the helicopter) by retreating-blade stall, and they rarely surpass 200 km/h. Autogyros are very easy to fly, I would say that far easier than fixed-wing...

Related: What are the uses of gyroplanes?

Image source
From the same website:

Zooming through the sky in something that looks like a flying chair, isn't for the faint-hearted.
These aircraft look like a cross between a small helicopter and an upside-down lawnmower.

Autogiro's are used by bush pilots for mustering in Australia, due to their versatility and relatively low cost. And are also used for police patrolling in Tomball Tx.
Image source
Advantages of the autogiro:

Versatile and small.
It can fly very slowly, safely. From this site:

Gyros cannot stall because the rotational speed of their rotors, and hence their lift, is not dependent on forward speed. At zero forward airspeed a gyro will slowly descend, to a survivable landing.

It can land anywhere.
It can takeoff from almost anywhere if the jump start is used: the engine spools up the horizontal rotor, then after take-off drives the propeller only.
Less complicated than helicopters, resulting in lower acquisition cost - much less complicated rotor hub.
Lower running costs than helicopters.

The disadvantages:

They are classified as recreational aircraft, with implications on licensing and oversight.
Some statistics show them to be more dangerous than most recreational aircraft. From this website:

Aviation statistics show gyrocopters to be more dangerous than most recreational aircraft. An Australian Transport Safety Bureau report on accidents between 2004 and 2013 found half of all gyrocopter accidents to be fatal, with the aircraft responsible for the worst fatality rate per hours flown.

This can also be because non-fatal accidents with autogiros are under-reported.
Licensing (in Australia) is done through the sports association, which implies no access to commercial use of gyrocopters.

The simple autogyros shown above are not built for speed with their un-aerodynamic shape. Power to overcome parasitic drag scales with (engine power)$^3$, and aircraft designed for high speed have a much more streamlined shape than the gyrocopter has. From this site:

Gyroplanes have somewhat different flying and performance characteristics to fixed wing aircraft. They have high drag and are therefore relatively slow, with typical cruise speeds of between 50 and 60 kt. The gyro’s advantage is not top speed but its ability to fly slowly, which is why under the name autogyro they were developed in the 1930s as a precursor to the helicopter.

But the flying armchairs for sheep mustering are not the only types of autogiro's. The fastest helicopters have a vertical thrust propeller as well:
Image source
Technically not an autogiro because the main rotors are driven, but the thrust propeller helps relieving the main rotor forward thrust. The Fairey Rotodyne was a pure autogiro, aerodynamically shaped, and had a maximum speed of 166 kts. Further speed increase is only possible when applying coaxial rotors like the X-2, to remedy instability due to retreating blade stall.
Image source

It's true that you can land a gyro almost anywhere, but taking off again may be impossible unless there is enough space for a take-off run. Jump-start autogyros do exist, but they are exceedingly rare and no longer series-built...

“Autogyros are not built for speed with their un-aerodynamic shape. Power to overcome parasitic drag scales with (engine power) cubed”. But this is true of any airframe design.


text:
What is the difference between SUA (Special Use Airspace) and FUA (Flexible Use Airspace)?
The EUROCONTROL Concept of the Flexible Use of Airspace (FUA) is that:

an ASM concept currently applied in the ECAC area and based on the fundamental principle that airspace should no longer be designated as
  either pure civil or military airspace, but rather be considered as
  one continuum in which all airspace user requirements have to be
  accommodated
an airspace management concept applied in the European Civil Aviation Conference area on the basis of the “Airspace management
  handbook for the application of the concept of the flexible use of
  airspace” issued by Eurocontrol

EUROCONTROL also defines Special Use Airspace (SUA) as:

Airspace of defined dimensions identified by an area on the surface of
  the earth wherein activities must be confined because of their nature
  and/or wherein limitations may be imposed upon aircraft operations
  that are not a part of those activities.

Question: what is the relationship exists between SUA and FUA? Can they be considered as the same thing? Is SUA a special type of FUA or vice-versa?
What is the difference essentially?

You can think of it as flexible use of special use airspaces.

Flexible Use of Airspace is a concept rather than a part of airspace (which SUA is), where the whole airspace is used for both restricted and unrestricted (civil) activities. According to FAA:

• The FAA version of Flexible Use of Airspace (FUA) is called “Joint Use Airspace”
• Although not defined as such, the domestic United States could be labeled as being completely FUA airspace.

The Special Use Airspace (SUA) is a portion of the Flexible Use Airspace (FUA), in which unrelated activities have been restricted:

In accordance with the FUA concept, Special Use Airspace is a defined portion of airspace which may be temporarily reserved for specific activities which require the restriction or segregation of other traffic.

Basically, in a FUA, SUA will be a temporary restriction (rather than a permanent one). Eurocontrol also says the same thing:

The EUROCONTROL Concept of the Flexible Use of Airspace (FUA) is that:
Airspace is no longer designated as purely "civil" or "military" airspace, but considered as one continuum and allocated according to user requirements.
Any necessary airspace segregation is temporary, based on real-time usage within a specific time period.

Related concept is Advanced Flexible Use of Airspace in which SUA is allotted in real time.

An airspace management concept in which airspace is managed as a single entity and in which there are no fixed structures and airspace reservations for special airspace activity are allocated in real time.


text:
Where did the US 'N' registration letter come from?
Why is the first letter in US registration numbers 'N'? Other countries seem to use a letter significant to their country, for instance 'G' for Great Britain, 'F' for France, 'D' for Germany (presumably for Deutschland), and 'JA' for Japan.
Does 'N' have any significance at all?

The origins of this can be traced back to CINA (the Commission Internationale de Navigation Aerienne / the Convention for the Regulation of Air Navigation) established as part of the Paris Peace Conference immediately after WWI.
A part of this conference was the adoption of a system of international aircraft identification still in use today. The U.S. delegation was allocated "N".
A number of theories exist as to why "N" was chosen specifically, and there's a great article about it over at the American Aviation Historical Society.
Theories range from, the largest number of states begin with the letter N; to the recognition of the development of wireless communications by the U.S. Navy that had been using "N" as the prefix to its station call-sign identifiers since 1909.

Joint-largest number of states. There are 8 M's and 8 N's.

And the ITU link is the one generally accepted as being the correct one. And the only one that makes sense.

I'm building this post expanding on Danny Beckett's information, give him credit :-)

Short answer:
Since 1919, aircraft registration is based on international prefixes allocated by ITU to radiotelegraphic stations rather than on a specific system. N prefix was allocated to US by ITU in 1919, along with KDA-KZZ and W. The choice for this system for aviation matters is materialized in the Chicago Convention since 1944, and previously in the Paris Convention it extended.
The question boils down to: Why did ITU allocate N and W (plus KDA-KZZ) to the US? instead of U, US or USA? The answer is likely US was not so involved in these matters at the beginning and choices were limited when the interest came:

CINA (which would become ICAO in 1947) and ITU, both at the origin of the prefix system, are also two of the many international peaceful initiatives by European governments to prevent recurrence of conflicts in the region.

ITU membership was reserved to States. Telegraph in the US was considered a matter of private companies, so US Government participation to ITU was not significant in the early years.

US will send an observer to ITU in 1875, 10 years after its creation, and US did agree in 1903 in Berlin that radiotelegraphy between ships and shore would be free to competition (read: Marconi stations would answer non-Marconi stations), but refused to ratify ITU radio regulations (RR) until 1949.

I guess founding members of ITU helped themselves with prefixes, and when the others came they had indeed fewer choices. Whole blocks (single letter prefix) have been allocated to actual or expecting-to-be empires: Germany (A, D), GB (B, G, M, and V for the overseas colonies), France (F), Italia (I), Japan (J), US (N, W), Russia (R).
In the end what was named the "call letters" had to be renamed the "call signs" with the addition of figures, like "2" for Guernsey or "9K" for Kuwait.
Historical milestones in international aviation vs international communication

1865: International Telegraph Union (ITU) created in Europe to handle telegraph matters.
1875: Saint Petersburg Telegraphic Convention.
1906: ITU Berlin Radiotelegraphic Convention. First convention about radiotelegraph.
1912: ITU London Radiotelegraphic Convention
1913: The International Bureau of the Telegraph Union in Berne releases a list of prefixes for radiocommunication. N and other blocks allocated to US. N has no specific meaning, it just happened to be available.
1913: US Department of Commerce makes the International Bureau list applicable in the US, in particular to ships and coastal stations.
1913: First passenger-carrying airline
1919: Paris Convention on ATC, prepared by CINA, deals with how allies, defeated, and other countries aircraft can be regulated, e.g. can they fly over another country? Part of this is the adoption of the national radio prefixes for aircraft registration and marking too. US did not ratify the convention.
1927: Washington Radiotelegraphic Convention, the aircraft marking system based on ITU proposal is agreed (is today Annex 7 of the Chicago Convention).
1929: Warsaw Convention sets aviation-related liability laws prepared by Paris-based CITEJA, Comité International Technique des Experts Juridiques Aériens.
1932: ITU is renamed International Telecommunication Union (Madrid Convention)
1944: Chicago Convention extends Paris Convention and creates PICAO, a temporary organization to replace CINA
1947: ITU is now part of UN
1947: PICAO becomes ICAO, ICAO is part of UN.

Agreement on radiotelegraph practices and station callsigns
At the beginning of the 20th century, radiotelegraphy ("wireless") was a new technology and communications with ships were handled using several variants of Morse code, Q-Code and protocols, by commercial operators like Marconi, according to their own practices. Even uniqueness of radio station identifiers among all involved stations was not guaranteed.
Such a technology used worldwide required some international agreements on important matters like frequencies, techniques, codes, station identifiers, etc. These agreements were obtained through formal radiotelegraphic conventions, in particular:

Berlin Radiotelegraphic Convention in 1906
London Radiotelegraphic Convention in 1912

These conventions established standards to be implemented by involved stations, of which a standard use of station identifiers (the "call letters").
Agreement on country prefixes for callsigns
Station identifiers had still to be delivered by different States, an agreement had to be found to ensure they would be unique. In April 1913 the International Bureau of the Telegraph Union released a list of national prefixes for station identifier allocation by States. This list was made official in the US by the Department of Commerce, in a document titled "Radio Call Letters", in May 1913:

The Service Regulations of the Berlin and London Radiotelegraphic Conventions provide that the call letters of stations in the
international system must each be formed of a group of three letters
which shall be distinguishable from one another.

The London International Radiotelegraphic Conference made a partial allotment of call letters among nations which signed the convention
and the International Bureau at Berne, with the consent of such
nations, has modified and added to this assignment of call letters by
circular of April 23, 1913.

The distribution of call letters among nations thus authorized is printed below for the guidance of operators of all stations, ship and
shore, of the United States.

Followed a list of international prefixes:

Q was reserved to prevent confusion with Q-Code sequences. The conventions also standardized this code, reserving QAA-QNZ to aviation and QOA–QQZ to marine.

Including KDA-KZZ, N, W for the US. N and W were possibly selected because they were the unassigned whole blocks at the time, while U, a natural choice, was already shared between France/Austria/Hungary/Bosnia-Herzegovina. Just a guess.

US DoC then spit this allotment: N for Government stations, W for stations of the Pacific (and some for the US Army), and KDA-KZZ for stations of the Atlantic.
Commission Internationale de Navigation Aérienne
The Commission Internationale de Navigation Aérienne (CINA) has been the precursor of ICAO. Its role was to help build international air navigation services.
To handle aircraft, ATC needed to identify them, and aircraft had to be registered at the State level, so that they could be linked to some legal system. CINA proposed to use a process similar to the one for radio stations, and to reuse the same ITU prefixes, with some minor adjustments (like adding a hyphen after the prefix) but also some changes, e.g. Germany was removed from the countries with a prefix:

Source

This proposition was adopted at Paris Convention in 1919. Paris Convention would later be extended by Chicago convention, and CINA replaced by ICAO, an organization of the UN. The prefixes would be updated in 1929 to the ones used today.
International Telecommunication Union (ITU) still maintains the list of prefix blocks in Appendix 42 of the Radio Regulations, and ITU is a full member of ICAO (without vote right).

More in Annex 7 – Aircraft Nationality and Registration Marks (commented here)

"W for stations of the Pacific (and some for the US Army), and KDA-KZZ for stations of the Atlantic". Are you sure about that? Assuming that this also relates to radio/TV stations, it's "W" east of the Mississippi and "K" to the west. WTHR in Indiana and KIFI in Idaho, for example. Could just be me applying apples to oranges...

@FreeMan: If you look at this DoC document: "3d: W [...] for ship and coast stations on the Pacific coast and on the Great Lakes.", however I found something interesting, a list of stations at that time, which uses W for East as you said. So it seems there is an error in the DoC document.


text:
Why does the Beech Staggerwing have its low wing ahead of the high wing?
Why does the Beech Staggerwing have its low wing in ahead of the high wing? What are the aerodynamic ideas behind this? Other biplanes of the era had the opposite, low wing behind the high one.

looking at the picture (didn't knew the plane before) my impression is that it was a structural/ease-of-use choice and not an aerodynamic one

it lets you step in behind the wing instead of in front of the wing right next to the propeller

Notice the retractable landing gear? With a tail-wheel design, the main gear work best at or ahead of the center of gravity. The Staggerwing has a neat and efficient retractable main gear, that folds onto the lower wing. But the lower wing has to be farther forward than usual for this to work.
The two wings on a biplane interfere with each other, but the biplane structure is very strong and stiff for its weight. So the Staggerwing's design gives up some wing efficiency for strength and light weight. Then it gives-up a little more for the reverse stagger that puts the downwash of the lower wing right under what should be the high pressure under the upper wing.
However, the design gets good speed from the retractable gear reducing drag. And both wings, upper and lower, join the fuselage at 90 degrees, with no struts or other sources of drag. Aerodynamically, a very clean design. the Beech Bonanza looked more modern, but the Staggerwing had the same cruising speed, 176 knots, 202 mph. It also had a 450 hp P&W Wasp Junior, while the Bonanza got along with a 300 hp Continental.

wow, the performance comparison is really interesting!

Bill IV, the Staggerwing has both a strut between the upper and lower wings, and wire bracing between the wings.  See Google Images for photos that show this clearly.

Skip Miller, sorry, I wrote confusingly. The Staggerwing does have a single strut joining the wings out by the tip, and paired bracing wires in a double "X" between those struts and the sides of the fuselage. (See red plane at the top of this page) But the wings themselves join the fuselage cleanly, no struts in the airflow connect either wing to the fuselage. Compare to a Stearman, SPAD or Tiger Moth. The Bristol SB2 has struts from both lower and upper wing to the fuselage. By contrast, the Fokker DR-1's low and mid wings have no struts to the fuselage, Only the top wing needed them.

@BillIV: The aerodynamic efficiency is not really affected by staggering the wings. The Staggerwing arrangement is not inferior to one with positive stagger.

I'll get references, or lack of them, and get back to you.

Wikipedia (certainly not a proper source) suggests:

The Model 17's unusual negative stagger wing configuration (the upper
  wing staggered behind the lower) and unique shape maximized pilot
  visibility while negligibly reducing interference between the wings.

which itself is sourced from "The Beechcraft Biplanes". Sport Avaition. January 1961.
Normally as you state the choice is for the lower wing to be further back due to the negative pressure on top which you do not want interfering and reducing the effectiveness of the upper wing.

Ted Wells did this just for visibility. It turned out that is was also good for flight characteristics as the bottom wing would stall first causing the airplane to pitch forward thus preventing the top wing from stalling. It also provided the ability to retract the gear into the wing.

You should provide links and references for sources and further readings.

Addendum to @ManuH 's comment: promoting a book on amazon is not really ok.

@Federico I agree. Citing a book without an amazon link should make the job.


text:
What factors contributed to enable Rickenbacher to be found?
In Rickenbacher's autobiography he tells of his flight across the Atlantic in a B-17 after WWII. They ended up running out of gas as I recall, making a water landing in the middle of the ocean. They were stranded for about a month before being rescued.
In the early post-WWII era, how was it that they were able to find him at all? What factors contributed?

atlantic current is know to go to the west of the UK a much busier area

Actually it was during WW2 and it was the Pacific:

One of Rickenbacker's most famous near-death experiences occurred in October 1942 [...] The bomber strayed hundreds of miles off course while on its way to a refueling stop on Canton Island and was forced to ditch in a remote and little-traveled part of the Central Pacific Ocean.

I am not familiar with the episode, and this source is currently not working for me, but from the little information that the wiki reports, I have the impression that the main factor has been pure luck.

ah, ok, it's been a while since I read his autobiography. ;)

@flyingfisch no problem. I actually find amusing the wiki's phrasing One of Rickenbacker's most famous near-death experiences must have been an hell of a life

yeah it was, he got into many near-death incidents, starting at age 9 IIRC. You should read the autobiography ;)

@flyingfisch I will, thanks for bringing it up :)


text:
Why aren't cell phones allowed to be used in aircraft?
What problems do cell phones cause? How is using a cell phone in the air different than using it from the ground?

This question covers cellular data, but the answers apply equally to cell phones.

...and sadly, one of the few remaining places where it's possible to be surrounded by strangers not yacking into their phones seems to be heading for extinction.

@Simon Dude! we are living in the 2010's! This is the decade of txting! Actually, I find the lack of people yacking and the increase of people texting, often in the same room, even at the same table disturbing.

Ground station for cell hone are designed to communicate with a phone that is not too high (let's say few tenth of meters abouve the ground). If the cell device is higher, base station cannot reach it (a base station emit mainly horizontally)

SKYbrary: "Although there was never any documented evidence of risk to flight safety, for many years, the “Precautionary Principle” was applied." Looking at Nasa list of incidents, I see nothing seriously argued. This alleged danger looks like an archaism, part of the same batch than useless underwater locator beacons and aircrafts lost at unknown locations.

Why aren't cell phones allowed?/ What problems to cell phones cause?
Most of these cases apparently originate from a series of unexplained events in the 90s which were believe to be caused by electronic gadgets, hence the ban. This was when cell phones appeared, and there was an aggressive ban as a result. NASA has a list which includes some of these and makes for an interesting read.
Now however an increasing list of airlines is allowing them again, depending on jurisdiction. FAA is working on allowing it from a regulatory standpoint. Bear in mind that phones were probably much more powerful back when these rules were introduced.
Mythbuster (episode 6, 2006) did a test on this by sticking in a powerful transmitter into a business jet. Their website gives a slightly different version: It was the FCC, not the FAA, who led the way for this regulation:

Explanation: Never mind what the chatterbox in the seat next to you
  says about cell phones messing with plane navigation — those metallic
  birds are built airtight against foreign signals and operate on
  entirely different frequencies than cell phones.
So why all the fuss about phones? When you make a call at 10,000 feet,
  the signal bounces off multiple available cell towers, rather than one
  at a time. That means too many phone-happy jetsetters might clog up
  the networks on the ground, which is why the Federal Communications
  Commission (FCC) — not the Federal Aviation Administration (FAA) — banned
  cell use on planes.

How is using a cell phone in the air different than using it from the ground?
Is it sensible to allow them on aircraft? To quote one of the above sources:

although the FAA recommends that you still switch to airplane mode
  because you’re not going to get a signal 30,000 feet in the air — the
  only hit you’ll take is a dead battery when you land.

So it's probably not a huge loss if you're flying commercial aircraft at 10,000 meters. In any case, you can't expect a service like you do on the ground, unless like Lnafziger says that the aircraft is equipped with a cellular basestation called a Picocell, illustrated below:

(source: wordpress.com)

Is it against FAA regulations to use a cell phone while in the air?

@flyingfisch http://www.wired.com/autopia/2013/10/faa-ban-lifted/

Note that where it is allowed, They are using special equipment on board the aircraft (picocells) in order to solve the issues that you mention here.  They don't just suddenly allow the use where they didn't before for no reason.

@Lnafziger ah, thanks for clarifying. :)

Also note that the same MythBusters episode tested a GA avionics panel as well (the question does not specify airliners, but perhaps it was implied?), and if I recall correctly it made the VOR go bananas.

And by GA, i meant smaller, c172/pa28 type of aircraft. The jet they tested was probably GA too. I'll just go ahead and punish my own stupidity by smacking myself in the face.

The ban on cell phones has nothing whatsoever to do with potential interference with the aircraft, at least not in the U.S. The former ban on use of any personal electronic device during critical flight phases (taxi, takeoff, and landing) was due to that reason, but that ban has been lifted. To answer flyingfisch's question, no, it is not against FAA regulations to use a cell phone while in the air. It is, however, against FCC regulations due to interference with the cell network, not the aircraft, as the quote in this answer from the Mythbusters website correctly points out.

The picture in your post broke

Wow, that  NASA report was a real eye opener!  The vast majority of cases related to lithium batteries and their bad behavior.

The WordPress link at the end no longer works; that WordPress blog has been deleted.

I am a private pilot, and sometimes, the cellphones cause interference with the radio. Most of the time, cellphones are not a problem, but once in a while, I have to turn mine off because it is causing parasitic noises on the radio.
I don't know how bad it would actually be in a passenger aircraft. But if it is similar to my experience in small aircraft, then it would be easier to just ban cellphones, rather than have a little light (like the seatbelt light) indicating when you can or can't use your phone.

bad comparison. Not only are the requirements for shielding of equipment a lot less strict on light aircraft than they are on airliners, the equipment is also located a lot closer to the people on board in light aircraft.

This also happens on Dash 8s.  Whilst I recognise the shift, I am still concerned.  Not with nav but with radio.  A friend of mine is an avionics trainer for a regional airline using Dash 8s.  Cellphone interference is often heard.  What I don't understand is why people aren't concerned with "Mistral 37, avoiding action immediate crack fizzle pop turn".  Was that left or right?  I am confused.

These noises were an artifact of early 2G/GSM networks; the only thing they really interfered with was hearing what ATC was saying. It generally doesn't happen on 3G/4G networks, but in the air you might occasionally only get a 2G connection if anything. Filtering it is a solved problem...

I believe GA pilots use phones a bit, which might not always be a good idea, esp. solo in an R22: https://www.taic.org.nz/inquiry/ao-2008-007

Cell networks require that cell towers be able to distinguish between signals coming from phones which are within the zones served by the towers, and those coming from phones in more distant zones.  There are nowhere near enough radio frequencies or time slices to allow every zone in the US to have its own combination of frequency and time slice.  In general, however, since the amount of signal received from a phone that's transmitting at a particular power level will drop off in proportion to the square of the distance, arranging zones so that the closest phone outside of a zone that will use a particular frequency/time-slice combination will never be less than be 3 times as far away as the furthest phone within the zone will generally make it possible for towers to receive the signals from phones within their service zones without too much interference from phones in other zones.
If a phone which is on a plane, 30,000 feet above the surface, transmits a signal which is loud enough to be heard by a tower serving a zone directly below, that same signal is likely to be received almost as well by towers for quite a ways around, some of which might have been expecting to use the same frequency and time slice to serve their own zones.  A cell network might be able to recognize the situation and assign frequencies and time slices in such a way as to avoid interference, but a single phone call from a plane might tie up ground-level bandwidth resources in multiple zones which, but for that call from the air, could each have used those resources to handle a separate call.

Welcome at Aviation.SE supercat. Interesting post. So you reckon the reason for disallowing cell phones on aircraft is to protect the cell network?

@DeltaLima: The FAA restricted them for fear of interference with airplane systems; the FCC because of interference with cellular networks.

Great clarification. Just adding that in case cell phones are authorized in planes, then a local base transceiver station (BTS) will be embedded on board and will be connected to a communication satellite. Planes are too fast for cell phones or BTS to maintain a link with a ground network (doppler shift and cell size), signal is attenuated by the cabin at best, and completely interrupted in most of the cases.

@DeltaLima Yes, the FCC banned them for this reason. Someone can correct me if I'm wrong, but I don't think the FAA ever actually had a ban specifically against cell phone use on board. They did have a ban on use of any electronic device during critical flight phases, but that was removed in 2013. The FCC ban remains, though, IIRC, they are studying removing it. Having said that, if they remove their ban, the DoT has said it may place its own ban on voice calls for the simple reason that passengers don't like listening to each other's phone conversations for hours on end.

@reirab: While some passengers may find phone usage annoying, others may value the ability to conduct business during what would otherwise be idle time.  Unlike smoke from the back of the plane which could end up circulating throughout the aircraft, chatty people in the back of the plane should not affect those elsewhere.  I would think the optimal solution would be for airlines to ask passengers whether they wish to use their phones, whether they wish quiet, or whether they don't care (perhaps offering free headsets, an extra snack, or some other perk to people who opt for the latter choice).

@supercat The airlines have asked passengers and apparently the response has been overwhelmingly opposed to allowing voice calls. See this press release from Delta which states that they will not allow voice calls, even if the government begins allowing them. I suspect that this opposition is especially true on longer flights where many people want to sleep.

@supercat While I suppose that having 'talking sections' is a possible solution, it would seem to be impractical to split the cabin up like that. I suppose if there is enough demand for it (and it gets regulatory approval,) some airline(s) will start offering it to cater to that market. Business travelers can still use e-mail, text messages, etc. without having to make voice calls, though.

@reirab: One of the brilliant things about free markets is that they can accommodate a wide variety of customer desires. If airlines can set their own rules, then they can react to the market if it is under-serving either people who want to talk or people who want quiet. If some airlines disallow talking entirely, then another airline who is presently attracting less than its share of quiet-seeking passengers may decide it makes more sense to go after the talkative passengers for whom other airlines aren't competing.

Apart from Manfred's answer, there are other factors in play:

Airlines make quite a nice income from the payphones in aircraft
Cellphone towers typically have pretty bad (if any) reception straight up, they're designed for ground level use after all which dictates the design of their antennae (an omnidirectional one is a lot less energy efficient)
People in border regions already complain about being hit by roaming charges when they set their phones to automatically switch networks. In an aircraft that'd be multiplied as those foreign towers might be in range from further inside a nation's borders (but see previous point as to why maybe not).
keeping a quiet cabin. As hinted, 150+ passengers all yacking away on their cellphones (and most people tend to shout into the darn things) make one hell of a racket. Bad marketing for the airline. Everyone wants every other passenger to be quiet and behave but want for themselves the right to do whatever they want, other passengers be darned.

@mins no, the alarmists still have to prove it's NOT safe. There's no way to ever prove there's no risk whatsoever, nor should you have to.

@mins 'Alarmist' is accurate in this situation. RF transmissions are not ionizing radiation. Additionally, assuming each of the flight crew members has their own personal cell phone (this seems like a reasonable assumption nowadays,) they would be getting far more total microwave radiation from occasional use of their own phone than they would from people on board, even if everyone were using their phone for the entire flight every time. This is because received power follows an inverse square law. Power decreases in proportion to the square of distance from the transmitter.

@mins Flight crews do have real radiation concerns, but cell phones aren't one of them.

@mins According to the American Cancer Society most such studies have found no link. Only a rather small minority of studies have suggested any link. As far as the comparison to microwave ovens, that's an apples-to-oranges comparison. Those things emit around 1,000 W. A cell phone connected to an on-board picocell is likely to be a few mW at most, perhaps less. Also, an aircraft cabin is a bit larger than a microwave oven.

What problems do cell-phones cause?

The FAA believes they could cause controlled flight into terrain.

Airlines have been ordered to replace or modify the cockpit display units fitted to hundreds of Boeing jets.
The US air safety regulator said that tests had indicated that mobile phone and computer signals could cause the screens to go blank.
...
"We are issuing this AD [airworthiness directive] to prevent loss of flight-critical information displayed to the flight crew during a critical phase of flight, such as an approach or take-off, which could result in loss of airplane control at an altitude insufficient for recovery, or controlled flight into terrain," it said.

From BBC

The use of cellular telephone radios remains banned on board aircraft in the U.S. It's banned by the FCC, though, not the FAA. The reason is that they (at least used to) wreak havoc on the cell network. The FCC is considering removing this rule, since it's largely outdated, but the DoT is considering placing its own ban on in-flight voice calls because most passengers have indicated that they don't want to hear people talking on the phone for several hours on end. Delta, for instance, has stated that they will continue to ban voice calls regardless of government regulations.

I am not myself a pilot, but I have a number of friends that pilot small private aircraft and use cellphones while in flight.  At altitudes of 10 or 12 thousand feet and speeds of 150 kph they work just fine.
People say they might 'interfere' with the aircraft.  But if the electronics in aircraft are really so susceptible to interference, why are you allowed to use cell phones in the airport?

Being a few inches or feet from the electronics is quite different from being a mile from them. Signal strength degrades with respect to the square of the distance from the transmitter, so a cell phone being used on board the plane would provide several orders of magnitude more interference to electronics on board than a cell phone being used in the terminal.

remember that airliners fly at 30-40 thousand feet, not 10 thousand. With the range of cell towers decreasing currently (the newest bands have a range of only a few kilometers) that puts the towers out of reach of modern phones located in airliners at cruise altitude.

I think you'll find that cell towers have enough problems supporting users on the ground with the spectrum available without having to point the antenna skywards in the hope of supporting flights.
The mention of a picocell on an aircraft would work but this requires expensive back haul over a satellite link and the time delays that incurs.

if we use a cellphone in an aeroplane it might disturb connection between the controlroom and plane and also between radar. which will be dangerous that it makes unable to connect the control room for an emergency.
i think this is the reason that cellphones are not allowed to use in planes

no, that's the stated reason. But it's hard to believe that the systems are that sensitive as it'd mean a terrorist could bring down an airliner by putting a working cellphone in his bag, put it in the overhead bin, and have someone call him while in the air...

@jwenting Actually, it's not even the stated reason. The stated reason is that hundred of thousands or millions of cell phones broadcasting at full power with each of them in line-of-sight to thousands of cell towers at any given point and moving at 600 mph at least used to wreak havoc on the cell network. It's banned by the FCC, not the FAA. However, it's my understanding that it doesn't matter as much to the current cell network, which is why the FCC is considering abolishing the rule.

However, even if the FCC gets rid of its rule, the DoT is considering banning the use of cell phones on board in order to keep passengers from beating up other passengers that are talking loudly and incessantly on their phones while others are trying to sleep. Studies have continually shown that most passengers want talking on phones in flight to continue to be banned. Delta has stated that, even if the FCC ban is lifted, it will continue to disallow voice calls due to customer and crew feedback.